Now that templates instantiations are no longer emitted for all modules that even blink at them they seem to break due to being linkonce (if compiled with any optimization level > 0), so let's give them weak linkage instead. The difference is that unreferenced linkonce symbols can be deleted, while weak symbols need to be preserved.

module complex1;

void main()
{
    cfloat cf1 = 3f + 0i;
    cfloat cf2 = 4f + 1i;
    cfloat cf3 = func();
    auto c1 = cf1 + cf2;
    auto c2 = cf2 - cf3;
    auto c3 = cf1 * cf3;
    auto c4 = cf2 / cf3;
}

cfloat func()
{
    return 3f + 1i;
}